1. Field of the Invention
The present invention relates to the field of hearing aids. The invention more specifically relates to hearing aids utilizing compensation for direct sound. The invention, more particularly relates to hearing aids having means for adjusting the hearing aid gain based on a rationale that takes into account the direct sound propagation around the hearing aid earpiece, and, still more particularly, respective systems and methods thereof.
2. Description of the Related Art
Hearing aids are adapted for providing at the users eardrum a version of the acoustic environment that has been amplified according to the users prescription. This is normally achieved by providing a device with a microphone, an amplifier and a miniature loudspeaker situated in an earpiece placed in the users ear canal. It is well known that there may be acoustic leaks around the earpiece. There may e.g. be a non-sealed fit or there may be a vent deliberately arranged in the earpiece for considerations about user comfort, e.g. for relieving the sound pressure created by the users own voice. Such leaks may cause a loss in sound pressure and they may allow sound to bypass the hearing aid to reach the ear drum.
WO-A1-2007045271 (PCT application PCT/EP2005/055305) titled “Method and system for fitting a hearing aid”, the contents of which are hereby incorporated by reference, provides a method for estimating otherwise unknown functions such as the vent effect and the direct transmission gain for an in-situ hearing aid. The derived estimate of the direct transmission gain represents the amplification of sound from the outside of the vent to the eardrum. These functions are used for correcting the in-situ audiogram, the hearing aid gain as well as the direct transmission gain according to the vent effect.
It is a widely known problem in hearing aid design that a hearing aid gain is often applied without taking into account the acoustic effect of the ventilation canal and/or a leakage path between the earplug of the hearing aid and the ear canal.
In hearing aids with open fittings or large ventilation canals, sound may propagate around the hearing aid earpiece, e.g. directly through the vent, to be superimposed onto the sound amplified by the hearing aid. In case these two sound signals are of similar amplitude, the summed signal may at certain frequencies be infinitely small if the relative phase between the signals is 180°. Such a phase disrupted signal has an unnatural rasping sound, and e.g. speech intelligibility may suffer as a consequence. The degree to which this is a problem depends on the individual hearing loss and the earplug. To the best knowledge of the inventors this problem has not been addressed in hearing aid fitting according to the prior art.
Therefore, acoustic effects of the ventilation canal and possible leakage paths between the hearing aid and the ear canal are still challenges in today's hearing aid fitting strategies.
Thus, there is a need for improved hearing aids as well as improved techniques for adapting the fitting rationale to take into account the direct sound propagation.